A vehicle, such as an automobile, is provided with systems, including a driving/braking system that transmits a driving force from a propulsive force generating source, such as an engine, to wheels or imparts a braking force, a steering system (steering control system) for controlling the steering control wheels of a vehicle, and a suspension system that elastically supports a vehicle body on wheels, as main mechanisms. Furthermore, in recent years, there has been known a vehicle provided with a variety of electric or hydraulic actuators to actively (positively) control the operations of the actuators according to a traveling condition of the vehicle or an environmental condition or the like rather than merely passively operating the systems in response to operations (man-caused operations) of a steering wheel (driver's wheel), an accelerator (gas) pedal, a brake pedal and the like performed by a driver, as shown in, for example, Japanese Patent Laid-Open No. 2000-41386 (hereinafter referred to as patent document 1).
Patent document 1 proposes a technology for determining the feedforward target value of a rear wheel steering angle on the basis of a front wheel steering angle, and for determining the feedback target value of the rear wheel steering angle on the basis of a difference between a reference state amount (a reference yaw rate and a reference lateral acceleration) and an actual state amount (a yaw rate detection value and a lateral acceleration detection value), and then making the rear wheel steering angle follow the sum of the target values. In this case, the reference state amount is set on the basis of a front wheel steering angle. Further, the parameters or gains of transfer functions of a feedforward controller, a feedback controller, and a reference state amount determiner are adjusted on the basis of an estimated value of a friction coefficient of a road surface.
However, the technology disclosed in the above-mentioned patent document 1 has been presenting the following inconveniences. The behaviors of an actual vehicle are subjected to the influences of a variety of disturbance factors, including changes in a friction coefficient of a road surface.
Meanwhile, it is practically difficult to sequentially generate optimum reference state amounts while taking all disturbance factors into account by using a vehicle model or the like. For instance, according to the one in patent document 1, although the parameters or the like of the transfer functions of the reference state amount determiner are adjusted on the basis of an estimated value of the friction coefficient of a road surface, actual vehicle behaviors are influenced by a variety of factors, such as variations in the characteristics of wheel tires, variations in the characteristics of the devices of a steering system and the like, estimation errors of friction coefficients, and modeling errors of a model for generating reference state amounts, in addition to the friction coefficient of a road surface.
Thus, according to the technology disclosed in patent document 1, there are cases where a variety of disturbance factors causes a reference state amount to be relatively significantly remote from a motion state of an actual vehicle. In such a case, an operation of a vehicle actuator is controlled on the basis of a control input that is unsuited to an actual vehicle behavior or the operation of the actuator is restricted by a limiter, thus making it difficult to ideally control the operation of the actuator.
Further, frequent use of a braking system of a vehicle by automatic control may cause the braking system (especially a brake pad, a brake disc or the like) to become hot. Hence, a brake manipulated variable in the automatic control is usually provided with a dead zone. As a result, however, a control system of the vehicle has been exhibiting a nonlinear characteristic, which inconveniently leads to deteriorated robustness of the vehicle control, as compared with the case where no dead zone is provided.
The present invention has been made in view of the background described above, and it is an object thereof to provide a vehicle control device capable of enhancing the linearity of a control system and enhancing robustness against disturbance factors or changes therein while carrying out control of the operations of actuators that suits behaviors of an actual vehicle as much as possible.